Satellite navigation systems provide a very attractive service for their global coverage and accuracy; but the accuracy is not sufficient for the safety of all useage applications, such as vessels in the harbor environment and aircraft during approach and landing. Position accuracy is limited by the ability of the satellites to provide correct and timely information on the satellite clock offset, orbital parameters, and propagation conditions. Satellites, furthermore, are not continuously monitored and in communications range of their terrestrial control center, and therefore do not provide sufficient integrity to notify users of malfunctions in a timely manner. Other limitations of concern are the availability of the minimum number of satellites needed for a fix, the orientation of satellites and the resulting geometric dilution of precision (GDOP), and the discontinuities associated with changing tracked satellites as they enter and exit the users' field of view.
GPS augmentation has accordingly been proposed for providing a communications channel to GPS users and transmitting data to improve the accuracy and integrity of the GPS, and may also include supplemental navigation service to improve availability and decrease discontinuities through timing relationships in the communications channel. An augmentation system for the provision of data to improve accuracy and integrity only is called Differential GPS (DGPS).
Two generic methods of such augmentation have been propsed in the literature and demonstrated (see, for example, the following articles; Johnson, G. "Results and Performance of Multi-Site Reference Station Differential GPS", Radice, J. T. & Wilson, R. "Standards For Maritime DGPS Reference Stations and Integrity Monitors, and Barboux, Jean-Pierre "An Assessment of Differential GPS Performance With Respect To User-Reference Separation", a Inst. of Navigation, Proceedings of 1993 GPS Meeting Sep. 22-24, 1994; and Alsip, Douglas H. CDR, et al "The Coast Guard's Differential GPS Program" Institute of Navigation, Proceedings of 48th Annual Meeting 29 June to 01 July 1994).
1) `Wide Area Augmentation` provides the user with data on each satellite clock offset, orbital parameters, and propagation conditions. This technique uses a specialized communications system to send data from multiple remote GPS monitor receivers to a central data processing facility. The central facility determines all of the various corrections for each satellite in view, and then sends the resulting messages over the specialized communications system to additional communications facilities for transmission to the users (a satellite link is proposed). The central facility also monitors the remote monitor sites for proper operation; and
(2) `Local Area Augmentation` which provides the user with data on pseudo-range corrections for each satellite in view of each of the multiple remote GPS monitor receiver sites, and then communicates directly from the monitor receiver sites (or nearby communications sites) to the users. Psuedo-range corrections combine all correction factors in a single term. The monitor sites are monitored both by internal checks and by a central facility for proper operation.
Either method can provide accuracies of the order of 5 meters or better; but there may be some degradation of local area augmentation beyond about 500 nm, as against the longer delays in the wide area augmentation system providing integrity messages and its lower communications reliability due to the multiple steps required in communications.
A much improved technique, in accordance with the present invention, is to use the Loran-C technology to augment SNS or GPS. Loran-C augmented GPS may also be either local area augmentation or wide are augmentation. In either case, the Loran-C signal is used as a communications signal and a supplemental navigation signal, timed to provide additional position information, and used to smooth user position and velocity data between augmentation data messages, and between adding and deleting satellites available to the user position and velocity solution. It is intended, but not always necessary, that a GPS monitor receiver be located at the Loran-C station.